Obama: ISIS a 'cancer,' U.S. to continue operations in Iraq

NEW: United States attempted a rescue in Syria, Pentagon spokesman says

More Americans, including Steven Sotloff, held by ISIS, official says

"The United States will do what we must," President Obama says

Airstrikes resume near Mosul Dam, a day after video of U.S. journalist

(CNN) -- The United States launched a series of airstrikes Wednesday against ISIS forces in Iraq, the same day President Barack Obama vowed to act against the militant group following its beheading of an American journalist and its threat to kill another.

"The United States of America will continue to do what we must do to protect our people. We will be vigilant and we will be relentless," the President said in televised remarks.

Obama vowed justice for James Foley, calling his killing by ISIS, which refers to itself as the Islamic State, an act of violence that "shocked the conscience of the entire world."

The President's statement followed Tuesday's release by ISIS of a video that showed Foley's killing and carried a stark warning that a second American, believed to be journalist Steven Sotloff, would be killed, if the United States did not end its military operations in Iraq.

Whether Sotloff lives or dies, the executioner said on the video, depends on what Obama does next.

ISIS is believed to be holding a number of Americans, including Sotloff, a U.S. official told CNN. The official, who spoke on condition of anonymity, declined to say exactly how many Americans were being held or to identify them.

Also Wednesday, Pentagon spokesman Rear Adm. John Kirby said in a written statement that the United States attempted a rescue operation earlier this summer to free a number of American hostages held in Syria by ISIS.

"Unfortunately, the mission was not successful because the hostages were not present at the targeted location," he said.

Another U.S. official, speaking on condition of anonymity, told CNN that the mission was to rescue Foley and others being held at an undisclosed location.

If ISIS, also known as ISIL, hoped the execution video and threat would ease U.S. military involvement in Iraq, it failed.

Calling ISIS a "cancer," Obama said across the Middle East there has to be a common effort to put an end to the group.

"Friends and allies around the world, we share a common set of values that are rooted in the opposite of what we saw yesterday," he said. "And we will continue to confront this hateful terrorism and replace it with a sense of hope and civility."

As the President spoke, the Pentagon confirmed the United States carried out 14 airstrikes against mobile ISIS targets in the vicinity of the Mosul Dam, which Kurdish forces recaptured from the terror group with the support of American airpower.

Humanitarian operation underway

Obama ordered targeted airstrikes in Iraq this month to protect U.S. personnel and facilities, as well as minorities being brutalized by ISIS.

The campaign by ISIS in Iraq began in June when the group swept in from Syria, seizing Iraq's second-largest city of Mosul.

ISIS militants, who are Sunni Muslim extremists, have carried out brutal attacks on towns and villages as they've advanced across Iraq, targeting Iraq's Christians, minority sects such as the Yazidis, centered in the northern Sinjar area, and Shia Muslims.

The UNHCR estimates that 1.2 million Iraqis have been forced from their homes so far this year, including more than one hal million in the western Anbar province and a similar number in northern Iraq.

A plane carrying the first load of humanitarian aid as part of a multiday operation to help hundreds of thousands of displaced people in northern Iraq has landed in Irbil, the U.N. refugee agency said.

"It's the largest single aid push we have mounted in more than a decade," the U.N. High Commissioner for Refugees, Antonio Guterres, said.

The first Boeing 747 to land carried 100 tons of aid, he said. Three more flights will follow from Jordan into Irbil, the Kurdish regional capital, with the last on Saturday.

The airlift will be bolstered by deliveries made by road and sea over the next 10 days, with 175 trucks ferrying cargo from warehouses in Turkey, Jordan and Iran.

The shipments include thousands of tents, plastic sheets, kitchen sets and jerrycans, destined for families who fled with little more than the clothes on their back.

Kurdish Peshmerga fighters assemble at a shrine on Iraq's Mount Sinjar on Friday, December 19. The Kurdish military said that with the help of coalition airstrikes, it has "cleansed" the area of ISIS militants. ISIS has been advancing in Iraq and Syria as it seeks to create an Islamic caliphate in the region.

A Peshmerga fighter looks down at the body of an alleged ISIS fighter in Zummar, Iraq, on Thursday, December 18.

A Kurdish fighter stands next to a destroyed armored vehicle in northern Iraq on December 18. The vehicle was destroyed by an improvised explosive device placed by ISIS militants.

Peshmerga fighters stop to check a vehicle in Zummar on December 18 as they continue to battle ISIS fighters near the border with Syria.

Pro-Iraqi government forces guard a shrine in Balad, Iraq, on Monday, December 15.

A Yazidi woman displaced by ISIS militants tends to a fire Wednesday, December 10, at a shelter in Dohuk, Iraq.

A Kurdish child from the Kobani, Syria, area holds laundry at a refugee camp in Suruc, Turkey, on Monday, November 17. Tens of thousands of people have fled Kobani, known in Arabic as Ayn al-Arab, to escape ISIS.

Smoke rises from Kobani following airstrikes on November 17. The United States and several Arab nations have been bombing ISIS targets to take out the group's ability to command, train and resupply its fighters.

A Kurdish child from the Kobani area holds on to a fence at a refugee camp in Suruc on Sunday, November 16.

People in Suruc watch smoke rise near the Syrian border during clashes between ISIS members and armed groups on Thursday, November 13.

A bomb (upper left) falls on an ISIS position in Kobani during an airstrike by the U.S.-led coalition on Saturday, November 8.

Iraqi military forces take up position in Jurf al-Sakhar, Iraq, on November 8.

Fire and smoke rise from Kobani following airstrikes against ISIS on Thursday, October 30.

Iraqi special forces search a house in Jurf al-Sakhar on October 30 after retaking the area from ISIS.

Smoke rises during fighting in Kobani on Monday, October 27.

ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23.

An explosion rocks Kobani during a reported car-bomb attack by ISIS militants on Monday, October 20.

People watch Kobani from a hill near the Turkey-Syria border on October 20.

Kurdish fighters walk to positions as they fight ISIS forces in Kobani on Sunday, October 19.

A U.S. Air Force plane flies above Kobani on Saturday, October 18.

Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on Saturday, October 18.

Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.

Turkish police officers secure a basketball stadium in Suruc on October 14. Some Syrian Kurds have been held there since crossing from Syria into Turkey.

Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.

Smoke rises from Kobani on Sunday, October 12.

Syrian Kurds from Kobani stand outside the grounds of a refugee camp in Suruc on Saturday, October 11.

Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.

In this photo released by the U.S. Air Force on Saturday, October 4, a U.S. Navy jet is refueled in Iraqi airspace after conducting an airstrike against ISIS militants.

A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk on Tuesday, September 30.

Iraqi Shiite militiamen aim their weapons during clashes with ISIS militants in Jurf al-Sakhar on Sunday, September 28.

Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.

Syrian Kurds wait behind border fences to cross into Suruc on September 28.

Tomahawk missiles, intended for ISIS targets in Syria, fly above the Persian Gulf after being fired by the USS Philippine Sea in this image released by the U.S. Navy on Tuesday, September 23.

Turkish Kurds clash with Turkish security forces during a protest near Suruc on Monday, September 22. According to Time magazine, the protests were over Turkey's temporary decision to close the border with Syria.

An ISIS flag flies on the other side of a bridge at the front line of fighting between ISIS and Kurdish Peshmerga fighters in Rashad, Iraq, on Thursday, September 11.

U.S. Secretary of State John Kerry reads on a flight en route to Iraq on Wednesday, September 10. Kerry traveled to the Mideast to discuss ways to bolster the stability of the new Iraqi government and combat ISIS.

Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.

An Iraqi fighter jet flies over Amerli, Iraq, on September 3. Amerli had been under siege by ISIS militants.

Iraqi volunteer fighters celebrate breaking the Amerli siege on Monday, September 1. ISIS militants had surrounded Amerli, 70 miles north of Baquba, Iraq, since mid-June.

Kurdish Peshmerga forces stand guard at their position in the Omar Khaled village west of Mosul on Sunday, August 24.

Kurdish Peshmergas fight to regain control of the town of Celavle, in Iraq's Diyala province, on August 24.

Peshmerga fighters stand guard at Mosul Dam in northern Iraq on Thursday, August 21. With the help of U.S. military airstrikes, Kurdish and Iraqi forces retook the dam from ISIS militants on August 18. A breach of the dam would have been catastrophic for millions of Iraqis who live downstream from it.

Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.

A fighter with Kurdish Peshmerga forces battles ISIS militants near Mosul on Monday, August 18.

Peshmerga fighters inspect the remains of a car that reportedly belonged to ISIS militants and was targeted by a U.S. airstrike in the village of Baqufa, north of Mosul, on August 18.

Kurdish Peshmerga fighters fire at ISIS in Khazair, Iraq, on Thursday, August 14.

Volunteers of the Iraqi Red Crescent Society unload boxes of goods before distributing them August 14 to families who fled from ISIS.

From the flight deck of the USS George H.W. Bush, which is in the Persian Gulf, two U.S. fighter jets take off for a mission in Iraq on Monday, August 11. U.S. President Barack Obama authorized airstrikes against Islamic militants and food drops for Iraqis who are trapped by the militants.

Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.

Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.

Iraqi Shiite volunteers who have joined government forces to fight ISIS take part in a training session near Basra, Iraq, on Thursday, August 7.

Thousands of Yazidi and Christian people flee Mosul on Wednesday, August 6, after the latest wave of ISIS advances.

A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.

A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.

Iraqis who fled fighting in the cities of Mosul and Tal Afar try to enter a temporary displacement camp in Khazair on Wednesday, July 2.

Peshmerga fighters check cars at the entrance of a temporary displacement camp in Khazair on Thursday, June 26.

Kurdish Peshmerga take their positions behind a wall on the front line of the conflict with ISIS militants in Tuz Khormato, Iraq, on Wednesday, June 25.

Peshmerga fighters clean their weapons at a base in Tuz Khormato on June 25.

New army recruits gather in Najaf, Iraq, on Wednesday, June 18, following a call for Iraqis to take up arms against Islamic militant fighters.

An MH-60R Sea Hawk helicopter lands on the aircraft carrier USS George H.W. Bush in the Persian Gulf on Tuesday, June 17.

Newly recruited Iraqi volunteer fighters take part in a training session in Karbala, Iraq, on June 17.

Members of ISIS prepare to execute soldiers from Iraq's security forces in this image, one of many reportedly posted by the militant group online. CNN cannot independently confirm the authenticity of the images.

Iraqi men chant slogans outside of an army recruiting center to volunteer for military service Thursday, June 12, in Baghdad.

Kurdish Peshmerga forces, along with Iraqi special forces, deploy their troops and armored vehicles outside of Kirkuk, Iraq, on June 12.

Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.

Civilians from Mosul escape to a refugee camp near Irbil, Iraq, on June 10.

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Photos: The ISIS terror threat

ISIS loses control of Mosul Dam

UNHCR: Iraq aid mission one of largest

Christian village evacuates ahead of ISIS

More US troops to Iraq?

As the massive aid operation swings into gear, fierce fighting continues between the militants and Kurdish forces for control of northern Iraq.

On Tuesday, U.S. airstrikes helped Kurdish and Iraqi forces take control of the key Mosul Dam, fighting back the militants who had seized it.

The stakes were huge for the millions of Iraqis who live downstream from the dam, the largest in the country, amid fears that if it were breached floods would have threatened lives in Mosul and downriver in Baghdad.

Now that the dam is cleared of ISIS militants, Iraqi forces are moving to grow their area of control, the Defense Department said.

The Pentagon is considering a State Department request for more U.S. troops to protect U.S. government personnel in the Baghdad area, a U.S. official confirmed to CNN.

If approved, the troops would number less than 300, said the official, who spoke on condition of anonymity.

"There is a request by the State Department for additional security personnel in and around Baghdad," the official said. The official would not say where the troops would go, but there are two publicly known locations for American personnel working in Baghdad -- the U.S. Embassy and Baghdad International Airport.

The official insisted that there was no specific threat that led to this request, which Defense Secretary Chuck Hagel would have to approve.

The troops would be in addition to the hundreds of American military advisers already in the country advising Iraqi troops in their fight against ISIS.